A number of medically recognized techniques are utilized for cataractous lens removal based on, for example, phacoemulsification, mechanical cutting or destruction, laser treatments, water jet treatments, and so on.
The phacoemulsification method includes emulsifying, or liquefying, the cataractous lens with ultrasonic power and then removing the emulsified material out of the eye. A phacoemulsification system 5 known in the art is shown in FIG. 1. The system 5 generally includes a phacoemulsification handpiece 10 having a hollow needle 15 at the distal tip (shown within the anterior chamber of the patient's eye 1) that is ultrasonically vibrated to emulsify the cataractous lens within the patient's eye 1. The system 5 typically includes an electronic control interconnected with handpiece 10 by an electric cable for powering and controlling the piezoelectric transducer. The handpiece 10 further includes an irrigation port 25 proximal to the distal tip of the needle 15 for providing irrigation fluid from an irrigation source 30 to the handpiece 10 via an irrigation line 35, and an aspiration port 20 at the distal tip of the needle 15, which is coupled to an aspiration pump 40 via an aspiration line 45 for aspirating fluid from eye 1 through the handpiece 10.
The hollow needle 15 is generally driven or excited by an applied AC voltage creating a piezoelectric effect in crystals. As is known in the art, this piezoelectric effect may provide either a longitudinal motion or a mix of longitudinal with transversal motions of the needle 15 to affect different desired cutting movements. Referring to FIG. 1b, the handpiece 10 may further include a vibrating unit 108 that is configured to ultrasonically vibrate the needle 15. The vibrating unit 108, which may include, e.g., a piezoelectric crystal, vibrates the needle 15 according to one or more parameters, such as frequency, pulse width, shape, size, duty cycle, amplitude, and so on. The motion of the vibrating unit 108 is amplified by a mechanically resonant system within the handpiece 10, such that the motion of the needle 15 is dependent, inter alia, upon the frequency at which the vibrating unit 108 is driven. The maximum motion of the needle 15 occurs at a resonant frequency. In part, this resonant frequency is dependent on the mass of needle 15 interconnected with the handpiece 10 that is vibrated by the vibrating unit 108.
Ultrasonic power of handpiece 10 generally consists of two parts: (1) a sine wave representing the voltage applied to the handpiece 10, and (2) the waveform representing the resultant current into the handpiece.
In purely capacitive circuits, there is a 90-degree phase angle between a sine wave representing the voltage applied to the handpiece 10 and the resultant current into the handpiece 10. In this case, the resultant current waveform leads the applied voltage waveform by 90-degrees (i.e., φ equals −90 degrees). Similarly, in purely inductive circuits, there is a 90-degree phase angle between the current and voltage such that the resultant current waveform lags the voltage waveform by 90-degrees (i.e., φ equals +90 degrees). Finally, for purely resistive circuits, φ equals 0-degrees.
Phacoemulsification handpieces typically operate in a range of frequencies between approximately 25 kHz to about 65 kHz. For each handpiece, an operational frequency window can be characterized by the handpiece impedance and phase. This frequency window is bounded by an upper frequency and lower cutoff frequency such that frequencies outside the upper and lower cutoffs have an electrical phase equal to −90-degrees. The center of this frequency window is typically the point where the handpiece electrical power reaches a maximum value.
The power transfer efficiency of handpiece 10 is defined by the formula (V*I)(cos φ). Accordingly, the most efficient handpiece power operating point occurs when the phase angle between the voltage applied to the handpiece and the resultant current into the handpiece is closest to φ degrees.
In order to maintain optimum handpiece power transfer efficiency, it is important to control the frequency of the phacoemulsification handpiece to achieve a phase value as close to zero (0) degrees as possible. However, controlling optimum power transfer efficiency is complicated by the fact that during operation, the phase angle of an ultrasonic handpiece is also dependent on the load of the transducer. This occurs through the mechanically resonant system including the handpiece needle 15. Specifically, when handpiece needle 15 comes into contact with tissue and fluids within eye 1, a load is created on the piezoelectric crystals with concomitant change in the operating phase angle.
As is well known, for these various surgical techniques, it is necessary to effect constant energy transfer into the tissue of the eye by the phaco handpiece regardless of loading effects. This can be accomplished by determining and measuring the phase angle at all times during operation of the handpiece in order to adjust the driving circuitry to achieve an optimum phase angle. For example, see U.S. Pat. No. 5,852,794 to Staggs et. al, filed Jan. 22, 1997 for a “Multiple Frequency Unambiguous Phase Detector for Phacoemulsification System” (“Staggs”), which is hereby incorporated by reference in its entirety by specific reference hereto. During phacoemulsification, it is possible to provide automatic tuning of a handpiece by monitoring the handpiece electrical signals and adjusting the frequency to maintain consistency with selected parameters.
Phase detection is the process of applying two electrical periodic signals of similar frequency into an electrical circuit that generates a phase difference, expressed in electrical degrees or time, between the two signals referenced to the same point in time. Conventional control circuitry, typically referred to as a phase detector, exists to measure the phase between the voltage and the current. Voltage generated by a phase detector is usually time averaged by an electronic circuit or sampled by an analog to digital (A/D) convertor and then averaged digitally. An averaged signal helps to reject some noise and can be read by a conventional voltage meter or used by a microprocessor. However, as mentioned above, phase shift is dependent on the operating frequency of the handpiece. Additionally, air and time delay in the measurement requires complex calibration circuitry in order to provide responsive tuning of the handpiece.
For use with a single frequency, the standard technique for measuring electrical phase has been to read a voltage that is proportional to the signal phase and also to frequency. This technique allows calibration for use with a single frequency. However, changing the frequency would cause the calibration data to be incorrect.
For multiple frequency uses, a typical approach has been to use two discrete analog integrators and an A/D convertor to generate phase data independent of the operating frequency. In particular, one integrator concurrently measures the period of a complete signal cycle while a second integrator measures the time between the voltage and current clock edges. Together the measurements are fed into an A/D convertor for use in a micro-controller (an example of such a system is described in U.S. Pat. No. 5,852,794 to Staggs et. al, filed Jan. 22, 1997 for a “Multiple Frequency Unambiguous Phase Detector for Phacoemulsification System”). Nevertheless, this system also involves some hardware calibration. For example, the two matched discrete analog integrators are susceptible to system performance variations due to component tolerances. Reducing variations involves matching component values as closely as possible in order to regulate drift of the individual circuits.
Accordingly, an improved system and method for measuring analog drive voltages and current for ultrasonic power control through phase manipulation is desirable.